GB2027038A - Method for preventing plasticizer bleeding on polyvinyl chloride shaped articles - Google Patents

Description

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GB 2 027 038 A 1

SPECIFICATION

Method for preventing plasticizer bleeding on polyvinyl chloride shaped articles

The present invention relates to a method for preventing or inhibiting the bleeding of a plasticizer on the surface of shaped articles of plasticized polyvinyl chloride resins and, in particular, to a .method for preventing bleeding of a plasticizer on the surface of shaped articles of polyvinyl chloride resins plasticized with a plasticizer having at least one aromatic nucleus in its molecule by the modification of the surface properties of the shaped articles.

Polyvinyl chloride resins are one of the most important classes of thermoplastic synthetic resins used . for manufacturing various kinds of shaped articles useful in a very wide field of applications. Shaped articles of polyvinyl chloride resins are classified into rigid type ones and flexible type ones according to the absence or presence of a substantial amount of plasticizer in the resin compositions. The rigidity or flexibility of shaped articles of polyvinyl chloride resin can be controlled by the incorporation of a plasticizer such as a phthalic ester and the thus plasticized polyvinyl chloride resins are fabricated into various kinds of flexible articles including films, sheets, synthetic leathers, tubes, hoses, bags, packings, covering materials and the like directed to uses in the fields of medical and sanitary wares,

cooking wares, wrapping materials for foodstuffs, insulation of electric wires and cables, materials used in agricultural plant growing, materials for buildings and the like.

One of the most difficult problems involved in the shaped articles of plasticized polyvinyl chloride resins is the so-called bleeding of the plasticizer contained therein whereby the plasticizer contained in the body of the shaped article migrates with time toward the surface of the article and is lost by evaporation into the atmosphere or by transfer to another body in contact with the shaped article. This phenomenon of bleeding of the plasticizer as well as of the other additives contained in the shaped articles is so remarkable that not only the attractive appearance but also various mechanical or physical properties are detrimentally affected giving rise to a shortened serviceable life of the articles.

This phenomenon of bleeding is especially undesirable when the shaped article is to be used for a medical purpose such as a bag or container of blood for transfusion or to be used in contact with foodstuffs, because the toxicity of the plasticizer and other additives present limitations to the application of polyvinyl chloride resins.

Various attempts have been made hitherto to decrease the bleeding of plasticizers and other additives on the surface of articles utilizing irradiation with ionizing radiations or ultraviolet light, corona discharge at a relatively high gas pressure, e.g. larger than 100 Torr, and treatment with chemicals. These prior art methods are to some extent effective for improving various surface properties of articles of polyvinyl chloride resins such as heat resistance, anti-solvent resistance, affinity with water, electrostatic charging, printability and the like but the effectiveness of these methods in preventing plasticizer bleeding is rather small. Moreover, it is sometimes unavoidable that certain properties of the shaped articles of polyvinyl chloride resins are disadvantageously affected by these methods.

For example, the irradiation with an ionizing radiation produces crosslinks not only in the surface layer but also in the body of the article with its high energy resulting in undesirable changes in the mechanical properties of the plasticized polyvinyl chloride resins. The irradiation with ultraviolet light is disadvantageous because it leads to coloring of the surface due to oxidative degradation taking place in the surface layer of the articles. The treatment with corona discharge is not practicable due to difficulty in the process control owing to the rather unstable nature of the discharge. Further, chemical means are not free from the problems of erosion by the chemicals or poor adhesiveness or durability of the coating compositions.

It was therefore desirable to provide a novel and effective means for preventing or inhibiting bleeding of plasticizers on the surface of shaped articles of a plasticized polyvinyl chloride resin or, in particular, of a polyvinyl chloride resin plasticized with a plasticizer which is a compound having at least one aromatic nucleus in its molecule or with a combination of plasticizers of which such an aromatic plasticizer is one of the components, to such an extent that the shaped article can be used in medical uses or in contact with foodstuffs and beverages with safety.

Thus, the present invention provides a method for preventing or inhibiting bleeding of a plasticizer on the surface of a chaped article of a polyvinyl chloride resin plasticized with at least 20 parts by weight, per 100 parts by weight of the polyvinyl chloride resin, of a plasticizer which is a compound having at least one aromatic nucleus in its molecule or of a combination of plasticizers including at least 10% by weight (of the total amount of the plasticizers) of a plasticizer which is a compound having at least one aromatic nucleus in its molecule, which method comprises subjecting the shaped article to a treatment with low temperature gas plasma.

The present invention is based on the unexpected discovery by the inventors that bleeding of a plasticizer on the surface of the shaped articles of plasticized polyvinyl chloride resins can be effectively prevented by the treatment with low temperature plasma only when the plasticizer used for plasticizing the polyvinyl chloride resin is a compound having at least one aromatic nucleus in its molecule such as an ester of phthalic acid or at least 10% by weight of the plasticizers is such an aromatic nucleus containing compound. On the contrary, beneficial effects can hardly be expected when 90% by weight or more of the plasticizers is a compound having no aromatic nucleus such as an ester of an aliphatic

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2 GB 2 027 038 A

dicarboxylic acid, e.g. dioctyl adipate and the like, or an ester of an aliphatic monocarboxylic acid, e.g. tributyl acetylcitrate and the like.

The polyvinyl chloride resins used in the fabrication of shaped articles usable in the method are not particularly limited to certain specific types of polyvinyl chloride resins and include homopolymers of 5 vinyl chloride of various degrees of polymerization as well as copolymers of vinyl chloride with one or more of copolymerizable comonomers insofar as the main component, say, 50% by weight or more, of the copolymer is vinyl chloride. The comonomers copolymerizable with vinyl chloride are well known in the art and are exemplified by vinyl esters such as vinyl acetate, vinyl ethers such as vinyiethyl ether, acrylic and methacrylic acids and esters thereof, maleic and fumaric acids and esters thereof, maleic ^ q anhydride, aromatic vinyl compounds such as styrene, vinylidene halides such as vinylidene chloride, acrylonitrile, methacrylonitrile, olefins such as ethylene and propylene, and the like.

On the other hand, the polyvinyl chloride resin composition for shaping the articles should contain about 20 parts by weight or more, per 100 parts by weight of the polyvinyl chloride resin, of a plasticizer which is a compound having at least one aromatic nucleus, e.g. a benzene nucleus, in its molecule and 15 exemplified by esters of phthalic acid such as dimethyl phthalate, diethyl phthalate, dibutyl phthalate, diisobutyl phthalate, diheptyl phthalate, di(2-ethylhexyi) phthalate, diisooctyl phthalate, di-n-octyl phthalate, dinonyl phthalate, diisodecyl phthalate, ditridecyl phthalate, dicyclohexyl phthalate, butylbenzyl phthalate, butyllauryl phthalate, or mixed ester of phthalic acid with alcohols having 7 to 10 carbon atoms and the like, esters of trimellitic acid such as trimethyl trimellitate, trioctyl trimellitate and 2q the like, phosphoric acid esters of phenols such as tricresyl phosphate, triphenyl phosphate and the like, ethylphthalyl ethylglycolate, butylphthalyl butylglycolate, diethyleneglycol dibenzoate and the like.

It is not always necessary that all of the plasticizers contained in the plasticized polyvinyl chloride resin are the above mentioned aromatic nucleus-containing compounds but it is sufficient that when these aromatic nucleus-containing plasticizers are used in combination with plasticizers having no 25 aromatic nucleus about 10% by weight or more of the total amount of the plasticizers is constituted by the aromatic nucleus-containing plasticizers in order to obtain satisfactory results using the treatment with low temperature plasma. The use of such non-aromatic plasticizers in combination with aromatic plasticizers is indeed recommended when improved mechanical properties of the shaped articles, especially at low temperatures, are desired.

2Q The plasticizers having no aromatic nucleus in the molecule are exemplified by esters of aliphatic dicarboxylic acids such as dioctyl adipate, diisodecyl adipate, dioctyl azelate, dioctyl sebacate and the like, esters of aliphatic monocarboxylic acids such as butyl oleate, methyl acetylricinolate, methyl esters of chlorinated carboxylic acids, methyl esters of methoxychlorinated carboxylic acids and the like, and related compounds. The amount of these non-aromatic plasticizers is, when used, limited to less than 35 90% by weight of the total amount of the plasticizers in consideration of the bleeding-preventing effect obtained by the treatment with low temperature plasma although it is recommended that the weight ratio of the aromatic plasticizers to the non-aromatic plasticizers is in the range from 20:80 to 80:20 in order to obtain a good balance of the low temperature properties and the bleeding-preventing effect of the shaped articles.

4Q The compositions of the plasticized polyvinyl chloride resins used for fabricating the shaped articles used in the present invention may be formulated with in addition to the above described plasticizers, various kinds of additives according to need including, for example, additives contributing to the improvement of properties such as heat stability, lubricity, weathering resistance and the like exemplified by metal soaps such as calcium stearate, zinc stearate, lead stearate, barium stearate, 45 cadmium stearate and the like, tribasic lead sulfate, dibasic lead phosphite, organotin compounds such as dibutyltin dilaurate, dibutyltin dimaleate, di-n-octyltin mercaptide, dimethyltin mercaptide and the like as a stabilizer and esters such as butyl stearate, aliphatic acid amides such as ethylene bisstearoamide, higher fatty acids such as stearic acid and polyethylene waxes as a lubricant, fillers, antioxidants, ultraviolet absorbers, anti-static agents, anti-fogging agent, pigments, dyestuffs, crosslinking 5Q aids and the like.

Furthermore, the compositions of the plasticized polyvinyl chloride resins may be formulated with certain kinds of high-polymeric rubbery elastomers to improve the properties of the shaped articles fabricated therewith. Suitable rubbery elastomers are exemplified by copolymers of ethylene and vinyl acetate, copolymers of acrylonitrile and butadiene, copolymers of styrene and acrylonitrile, copolymers 55 of methyl methacrylate, styrene and butadiene, copolymers of acrylonitrile, styrene and butadiene,

copolymeric elastomers of ethylene and propylene, copolymeric elastomers of ethylene, propylene and a dienic monomer and the like.

The shaped articles subjected to the method are not limited to a particular method of fabrication thereof and conventional fabrication techniques can be applied according toihe shapes of the desired gQ articles including extrusion molding, injection molding, calendering, inflation method, blow molding, compression molding, vacuum forming and the like. The shapes of the articles are also not limited although articles with complicated shapes, for example with concavity, may require a specific more elaborate technique in order to ensure uniformity in the effect of the treatment with low temperature plasma.

g5 The shaped article of plasticized polyvinyl chloride resin described above is subjected to a

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GB 2 027 038 A 3

treatment with low temperature plasma. Low temperature plasma is well known in the art as gaseous atmosphere full of electrically charged species where the temperature of the gaseous atmosphere is not excessively high in comparison with the ambient temperature irrespective of the energies of the charged species perse. Low temperture plasma is produced mainly by glow discharge in a gaseous atmosphere g at a pressure in the range from 0.001 to 10 Torr where the frequency of the electric power supply for the discharge is not critical ranging from direct current to the microwave region. In particular, a frequency of the so-called high frequency region is recommended due to the availability of generators with sufficient power output and the possibility of obtaining stable plasma discharge. For example, a frequency of 13.56 MHz or 27.12 MHz is recommended since these frequencies are relatively free 1q from statutory regulations for radio waves.

The shapes and arrangement of the electrodes are not critical insofar as a stable plasma discharge can be ensured within the space in which the surface of the shaped article is treated with, i.e. exposed to, the plasma atmosphere. Thus, a pair of inside electrodes, a pair of exterior electrodes and a coiled electrode may be used according to particular types of the apparatuses for plasma generation. The 1 g electrodes may be connected to the high frequency generator either by capacitive coupling or inductive coupling.

The intensity or power density of the low temperature plasma arid the time required for the plasma treatment are interrelated parameters but extreme difficulties are encountered in explicitly defining the power density of low temperature plasma due to the very complicated nature of the plasma discharge 2Q which is presently not understood. Therefore, the best approach is to say that the time for the plasma treatment is determined in advance by a careful preparatory experiment in which several parameters including the electric power supplied are selected according to the specific purpose. With the power density obtained in most of the currently available apparatuses for plasma generation, a time from a few second to several tens of minutes is usually sufficient for obtaining the effect of the invention. In any 25 case, it is a least requirement that the surface of the shaped articles never undergoes thermal degradation by the heat evolved by the plasma discharge.

The other parameters to be taken into consideration in the plasma treatment are the kind of gaseous constituents and the pressure of the gaseous atmosphere. The pressure of the gaseous atmosphere within the apparatus for plasma generation should be maintained in a range from 0.001 to 10 Torr, preferably from 0.1 to 30 1.0 Torr, in order to ensure stability of the plasma discharge. The gas filling the apparatus under the above specified pressure is either inorganic or organic as exemplified by helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon monoxide, carbon dioxide, hydrogen, halogens, e.g. chlorine, and halogen compounds, e.g. hydrogen chloride, as well as olefins, e.g. ethylene and propylene, halogenated hydrocarbons, e.g. fluorocarbons, aromatic hyrocarbons, e.g. benzene, heterocylic organic compounds, 35 e-g- pyridine, organosilanes and the like. Among the above named gases, the inorganic gases are preferred to the organic ones due to the absence of coloration on the surface of the plasma-treated articles and formation of a powdery polymerized matter. In particular, helium, argon, carbon monoxide, carbon dioxide and hydrogen, especially, carbon monoxide, are preferred because of their higher efficiency (due to an unknown mechanism). These gases are used either singly or as a mixture of two or 4q more and, when a mixed gas is used, it is recommended that one of the components is carbon monoxide.

The shaped articles obtained by the above described procedures of the method of the invention have a markedly reduced tendency to plasticizer bleeding on the surface and do not transfer the plasticizer to another body in contact therewith and, in addition, the flexibility of the surface layer which 45 constitutes the barrier layer for the plasticizer is not lost because of the adequate density of crosslinks with good heat-sealability and weathering resistance as well as good mechanical properties, especially at low temperatures, such as tensile strength, resistance against scratches, impact strength and the like. Further, the shaped articles obtained by the method of the invention have excellent surface properties such as improved affinity with water, reduced tendency to being stained and good resistance against 50 oils and chemicals.

The following Examples will further illustrate the present invention. In the Examples parts are all parts by weight. Table 1 referred to in each Example is given after the last Example.

EXAMPLE 1. (Samples No. 1 and No. 2.)

A resin composition was prepared by intimately blending 100 parts of homopolymeric polyvinyl 55 chloride resin having an average degree of polymerization of about 1,000 (TK—1000, a trade name of Shin-Etsu Chemical Co., Japan), 40 parts of dioctyl adipate (hereinafter abbreviated as DOA), 10 parts of dioctyl phthalate (hereinafter abbreviated as DOP), 1.5 parts of calcium stearate and 1.5 parts of zinc stearate in a roller mill at 160°C for 10 minutes and the resin composition was fabricated by press molding at 165°C into a sheet of 1 mm thickness.

gO A 10 cm by 10 cm piece of the thus prepared sheet was placed on a lower electrode of 20 cm .diameter facing an upper electrode at a distance of 3 cm in an apparatus for plasma generation and low temperature plasma was generated by a high frequency power supply of 100 watts at a frequency of 13.56 MHz for 10 minutes while the pressure in the apparatus was maintained at 0.2 Torr by passing carbon monoxide gas with simultaneous evacuation with a pump.

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GB 2 027 038 A4

The resin sheets before the plasma treatment (Sample No. 1) and after the plasma treatment (Sample No. 2) were then each subjected to a test for the plasticizer bleeding in the manner described below. Thus, the resin sheet suitably cut was placed on the bottom of a cylindrical extraction vessel, in the case of Sample No. 2 with the plasma-treated surface facing upwards, and set so that 26 cm2 of the upper surface thereof alone came to contact with the extraction solvent introduced into the vessel, and 5 then 50 ml of n-hexane was introduced into the vessel with subsequent shaking at 37°C for 2 hours, the concentration of the plasticizers in the extract solution was determined by gas chromatography to give the results set out in Table 1 in mg/cm2.

EXAMPLE 2. (Samples No. 3 and No. 4)

The experimental procedures were the same as in Example 1 except that the formulation of the 10 plasticizers was changed to 25 parts of DOA and 25 parts of DOP. The extraction test with the resin sheet before the plasma treatment (Sample No. 3) and after the plasma treatment (Sample No. 4) gave the results set out in Table 1.

COMPARATIVE EXAMPLE 1. (Sample No. 5)

The experimental procedures were the same as in Example 1 except that DOP in the formulation of 15 the resin composition was omitted and, instead, the amount of DOA was increased to 50 parts. The extraction test with n-hexane for this sheet (Sample No. 5) gave the results set out in Table 1.

EXAMPLE 3. (Samples No. 6 and No. 7)

The experimental procedures were the same as in Example 1 except that the formulation of the plasticizers in the resin composition was changed to 20 parts of DOA and 5 parts of DOP and the 20

pressure of carbon monoxide during the plasma treatment was 0.5 Torr instead of 0.2 Torr. The extraction test with n-hexane was carried out with the resin sheets before the plasma treatment (Sample No. 6) and after the plasma treatment (Sample No. 7) to give the results set out in Table 1.

COMPARATIVE EXAMPLE 2. (Sample No. 8)

The experimental procedures were the same as in Example 3 except that DOP in the formulation of 25 the resin composition was omitted and, instead, the amount of DOA was increased to 25 parts; also the high frequency power supplied to the electrodes was decreased to 50 watts. The extraction test with n-hexane carried out with this sheet (Sample No. 8) gave the results set out in Table 1.

EXAMPLE 4. (Samples No. 9 and No, 10)

The experimental procedures were the same as in Example 1 except that DOA in the formulation 30 of the resin composition was replaced with the same amount of di(2-ethylhexyl) sebacate (hereinafter abbreviated as DOS) and the high frequency power supplied to the electrodes was increased to 150 watts. The extraction test with n-hexane was carried out with the resin sheets before the plasma treatment (Sample No. 9) and after the plasma treatment for 5 minutes (Sample No. 10) to give the results set out in Table 1. 35

COMPARATIVE EXAMPLE 3. (Sample No. 11)

The experimental procedures were the same as in Example 4 except that DOP in the formulation of the resin composition was omitted and, instead, the amount of DOS was increased to 50 parts. The extraction test with n-hexane was carried out with this resin sheet (Sample No. 11) to give the results set out in Table 1. 40

EXAMPLE 5. (Samples No. 12 and No. 13)

The formulation of the resin composition was the same as in Example 4 except that DOP in the formulation was replaced with the same amount of tri(2-ethylhexyl) trimellitate (hereinafter abbreviated asTOTM) and the treatment of the resin sheet with low temperature plasma was carried out in an argon atmosphere instead of carbon monoxide under a reduced pressure of 0.5 Torr and with high frequency 45 power of 150 watts for 5 minutes. The extraction test with n-hexane was carried out with the resin sheets before the plasma treatment (Sample No. 12) and after the plasma treatment (Sample No. 13) to give the results set out in Table 1.

COMPARATIVE EXAMPLE 3. (Sample No. 14)

The experimental procedures were the same as in Example 4 except thatTOTM in the formulation 50 of the resin composition was omitted and, instead, the amount of DOS was increased to 50 parts. The extraction test with n-hexane was carried out with this resin sheet (Sample No. 14) to give the results set out in Table 1.

EXAMPLE 6. (Samples No. 15 and No. 16)

The experimental procedures were the same as in Example 1 except that DOA in the formulation 55 of the resin composition was omitted and, instead , the amount of DOP in the formulation was increased

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to 50 parts. The extraction test with n-hexane was carried out with the resin sheets before the treatment with low temperature plasma (Sample No. 15) and after the treatment (Sample No. 16) to give the results set out in Table 1.

EXAMPLE 7. (Samples No. 17 and No. 18)

5 A resin composition composed of 100 parts of a homopolymeric polyvinyl chloride resin having an 5

average degree of polymerization of about 1,300 (TK—1300, a trade name of Shia-Etsu Chemical Co., Japan), 40 parts of DOP, 10 parts of tricresyl phosphate (hereinafter abbreviated as TCP), 1.5 parts of calcium stearate and 1.5 parts of zinc stearate was fabricated into a sheet of 1 mm thickness in the same manner as in Example 1.

10 The plasma treatment of the thus prepared sheet was carried out in the same manner as in .10

Example 1 except that the pressure of carbon monoxide was maintained at 0.4 Torr and the high-frequency power was 150 watts. The extraction test with n-hexane was carried out with the resin sheet before the plasma treatment (Sample No. 17) and the sheet after the plasma treatment (Sample No. 18) to give the results set out in Table 1.

TABLE 1

Plasticizer formulated, parts by weight

Plasma treatment

Plasticizer extracted, mg

Sample No.

DOA

DOP

DOS

TOTM

TCP

Gaseous atmosphere

Pressure, Torr

High-frequency power, watts

DOA

DOP

DOS

TOTM

TCP

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40

10

-

-

-

Not treated

85

20

-

-

-

2

40

10

-

—

—

Carbon monoxide

0.2

100

23

5

-

-

3

25

25

-

-

-

Not treated

50

53

-

-

-

4

25

25

-

-

-

Carbon monoxide

0.2

100

1.1

0.2

-

-

-

5

50

—

—

—

—

Carbon monoxide

0.2

100

93

-

-

-

-

6

20

5

-

-

-

Not treated

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10

-

-

-

7

20

5

-

-

-

Carbon monoxide

0.5

50

0.6

0.05

-

-

-

8

25

—

—

—

—

Carbon monoxide

0.5

50

53

-

-

-

-

9

-

10

40

-

-

Not treated

-

19

76

-

-

10

-

10

40

-

-

Carbon monoxide

0.2

150

-

5

20

-

-

11

—

—

50

—

—

Carbon monoxide

0.2

150

-

-

96

-

-

TABLE 1 (Continued)

Plasticizer formulated, parts by weight

Plasma treatment

Plasticizer extracted, mg

Sample No.

DOA

DOP

DOS

TOTM

TCP

Gaseous atmosphere

Pressure, Torr

High-frequency power, watts

DOA

DOP

DOS

TOTM

TCP

12

-

-

40

10

-

Not treated

-

-

83

18

-

13

-

-

40

10

-

Argon

0.5

150

-

-

15

3

-

14

-

-

50

-

-

Argon

0.5

150

-

-

103

-

-

15

-

50 *

-

-

-

Not treated

-

63

-

-

-

16

-

50

-

-

-

Carbon monoxide

0.2

100

-

0.02

-

-

-

17

-

40

-

-

10

Not treated

-

-

85

-

-

22

18

-

40

—

—

10

Carbon monoxide

0.4

150

—

0.07

-

-

0.2

8

GB 2 027 038 A 8

Claims (8)

1. A method for preventing or inhibiting bleeding of a plasticizer on the surface of a shaped article of a polyvinyl chloride resin plasticized with at least about 20 parts by weight, per 100 parts by weight of the polyvinyl chloride resin, of a plasticizer which is a compound having at least one aromatic nucleus

5 at least about 10% by weight of a plasticizer which is a compound having at least one aromatic nucleus g in its molecule or of a combination of plasticizers including at least about 10% by weight of a plasticizer which is a compound having at least one aromatic nucleus in its molecule, which method comprises subjecting the shaped article to a treatment with low temperature gas plasma.

2. The method as claimed in claim 1 wherein the pressure of the gas for the low temperature

1 q plasma is in the range from 0.001 to 10 Torr. . _

3. The method as claimed in claim 1 wherein the pressure of the gas for the low temperature plasma is in the range from 0.1 to 1 Torr.

4. The method as claimed in any preceding claim wherein the gas for the low temperature plasma is selected from helium, neon, argon, nitrogen, nitrous oxide, nitrogen dioxide, oxygen, air, carbon

15i monoxide, carbon dioxide, hydrogen, halogens, halogen compounds, olefins, halogenated hydrocarbons, i g aromatic hydrocarbons, heterocyclic organic compounds and organosilanes.

5. The method as claimed in any preceding claim wherein the gas for the low temperature plasma is an inorganic gas.

6. The method as claimed in claim 5 wherein the inorganic gas for the low temperature plasma is

20 carbon monoxide or a mixed gas containing carbon monoxide. 20

7. The method as claimed in claim 1, substantially as described in any of the Examples.

8. A shaped article when treated by the method of any preceding claim.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1980. Published by the Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.